DESCRIPTION (Applicant's Description): Our laboratory recently discovered a high incidence of myeloid malignancies in heterozygous Nf1 mutant (Nf1 +/-) mice treated with the commonly used chemotherapeutic agent cyclophosphamide (CP). The subject of this application is to utilize this novel murine model for translational and mechanistic studies of CP-induced myeloid leukemia. These goals will be pursued through three specific aims. In aim 1, we will test the hypotheses (a) that increasing CP dose intensity will increase the incidence of t-ML in Nf1 +/- mice, and (b) that the chemopreventive agent amifostine will partially protect CP-treated Nf1 +/- mice from t-ML. Aim 2 will test the hypothesis that inactivation of the normal Nf1 allele correlates with clinical evidence of t-ML in Nf in mutant mice, and will perform molecular analyses to characterize the mechanism of Np inactivation in CP- treated mice. We will also determine the frequency of Hprt gene inactivation in lymphocytes from control mice and from mice that have been treated with CP, amifostine, or with both agents. These experiments will allow us to ascertain if Hprt mutation rates provide a useful surrogate marker. CP is metabolized to acrolein and to phosphoramide mustard (PM). While PM is responsible for the anti-tumor effects of CP, recent data implicate acrolein as contributing to its leukemogenic properties. Resolving this question has important implications for designing cytotoxic agents with reduced mutagenic potential. Therefore, our final aim will involve determining the incidence of leukemia in cohorts of Nf1+/- mice treated with CP, with an analog that produces only acrolein, or with an analog that generates only PM. In addition, we will perform correlative studies as described under aim 2. These experiments will allow us to compare the leukemogenic effects of these two major metabolites of CP, as well as the type(s) of genetic lesions which occur at Nf1 in hematopoietic cells from mice that received these agents.